As is known, many liquid or pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by creasing and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a base layer, e.g. of paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, e.g. an aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; the web so sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a tube, which is fed vertically.
In order to complete the forming operations, the tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections.
More precisely, the tube is sealed longitudinally and transversally to its own axis.
Pillow packs are so obtained, which have a longitudinal seal and a pair of top and bottom transversal seals.
Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the packages are then filled with the food product and sealed. One example of this type of package is the so-called “gable-top” package known by the trade name Tetra Rex (registered trademark).
More specifically, the pillow packs comprise a parallelepiped-shaped main portion; and opposite, respectively top and bottom, end portions tapering from the main portion to respective sealing lines crosswise to the pack. Each end portion has substantially triangular flaps projecting from opposite sides of the main portion; and a low rectangular tab projecting from the relative sealing line.
Packaging machines of the above type are known, on which the pillow packs are turned into folded packages by automatic folding units.
Folding units are known, for example from the International Application No WO2008122623 in the name of the same Applicant, which substantially comprise:                a rotary conveyor which receives pillow packs to be folded at inlet station, conveys pillow packs to be folded along an arc-shaped folding path, and outputs folded packages at an output station;        a first folding unit which interacts with a bottom portion of the pack travelling along the folding path to perform a folding operation onto the packs;        a heating device for heating the flaps of the packs travelling along the folding path; and        a second folding device for pressing flaps of each pack travelling along forming path onto respective wall, as flaps cool.        
In greater detail, rotary conveyor comprises a plurality of angular-spaced conveying devices, which grip packs at inlet station, and feed them along a forming path to output station.
Each conveying device comprises two flat surfaces which face each other and cooperate, in use, respectively with a front and a rear wall of the main portion of the relative pack to be folded.
A need is felt within the industry for the maximum flexibility as regards the final shape of packages folded by the folding machine.
This is particularly so in the case of newly conceived packages which have a front wall bulging on the opposite side of a rear wall.